The contractile elements of cardiac, smooth and skeletal muscle are critically dependent on the intracellular concentration of calcium ion, for regulation of their contractile state. In turn, the intracellular concentration of calcium ion is controlled by a system of membranes, e.g., Sarcoplasmic Reticulum, which can be isolated and display in vitro Ca 2 plus ion transport properties. It is the aim of this research proposal to combine physiological, pharmacological, physical and chemical techniques to investigate the mechanism of Ca 2 plus ion uptake and release by these membranes, in order to: a) unveil rate limiting steps in the control of contractile state in vivo, b) investigate the possible involvement of this control mechanism in cardiovascular pathology, and c) study the mechanism of action of known and new pharmacological agents on this system. The proposed work involves isolation of intracellular membrane particles, structural observations by electronmicroscopy, measurement of transmembrane ion fluxes, determination of enzymatic activities, chemical modification and labeling of enzymatic sites, spectroscopic studies of structural parameters in spin labeled membranes, experiments with lipid bilayers and reconstituted membranes, and parallel physiological and pharmacological studies on contractile parameters of cardiac, smooth and skeletal muscle preparations.